1. Field of the Invention
The invention relates to the field of recording and reading of data by a beam of radiation which is focused to form a spot on a substantially planar recording layer in a record carrier or medium, the spot and recording layer being moved relative to each other in at least one direction parallel to the layer. Marks or pits (hereinafter referred to generally as marks) in the recording layer cause a change in a parameter describing return radiation which is either transmitted through, or reflected by, the layer. In order to maximize the density of data recording, it is desirable to make these marks quite small and to place them close to each other.
A common form of recording medium is a transparent disc or substrate on which a recording layer is formed or deposited, and data are later recorded in the form of marks formed by optical or magneto-optical processes. Usually such a disc is "pre-formatted," by incorporating a track pattern which defines a spiral track centerline, or a multiplicity of circular track centerlines, and divides the tracks into frames or segments. A common pre-formatting technique, especially useful with the instant invention, involves forming pits or bumps (hereinafter referred to as control marks) in or at a surface of the recording layer. These control marks are arranged at the beginning of each frame in a so-called servo field, which includes clock and address pits used to establish read clock synchronization and frame identification. Frequently it is possible to pre-format control marks which are smaller in size than the recorded data marks.
Tracking and data readout use an optical beam from a semi-conductor laser, focused into the smallest possible spot for reading the pits and recorded data marks. To accomplish this some form of focusing servo is always required.
Another technique for organizing control information and data is commonly referred to as "continuous composite servo" (CCS). Instead of distributing control information at the beginning of each frame, a relatively long header or preamble is formed at the beginning of each sector. The sector is of such length that it may be divided into frames, but the individual frames do not start with a complete servo field.
As recording density has increased, and the reading beam spot has shrunk in size, variations in the thickness and flatness of the medium, whether disc, card or tape, and unavoidable variations in the focusing system, have led to highly sophisticated schemes for optimizing the beam focus with respect to the recording layer.
2. Description of the Prior Art
The basic procedure for setting the focus for a reading beam has been to use a focus servo to optimize the beam reflected from, or transmitted through, the medium. This method involves controlling the focus drive to minimize an error signal based on the size or position of a spot or pair of spots on a detector. For example, U.S. Pat. No. 4,464,741 to Compaan teaches a focus detector which images stigmatized patterns of the beam spot onto a 4-quadrant detector. This requires that the disc have a sufficiently large blank or unrecorded area so that the beam spot has even reflectance around its perimeter. The same optical system is also used for detecting clock signals in a pre-recorded servo track, and tracking errors.
An improvement in data readout is obtained by the technique of focus offset, for example as described in U.S. Pat. No. 4,907,212 to Pharris and Schell. According to the invention disclosed therein, focus calibration involves comparing the value of a signal at an isolated mark with the signal at an immediately preceding or following mark position, and adjusting a focus offset signal added to the focus servo error signal until the difference between the signals at the mark and the vacant (no mark) position is maximized. The offset value which causes that optimum read signal is then used as a fixed correction to the focus servo during normal readout.
The method of the '212 patent uses reflectance in the vicinity of a servo mark, particularly pre-recorded clock signals. After read timing calibration has been conducted, a series of comparisons are made between the reflectance at approximately the high point of the received signal from the pre-formatted mark (pit) and that at another point a fixed distance away. In many circumstances this requires a technician to substitute a special disc for the one normally in use in a particular drive.
In one well-known optical data recording drive, the LD510 sold by Laser Magnetic Storage International Company, the medium is a 51/4 inch diameter disc having preformatted clock pits. Every time a new disc is inserted in the drive, and every time the drive goes into its calibration mode, the electrical focus-offset signal is automatically determined. The read beam is directed toward a track, and because of relative motion due to rotation of the disc about its axis, the beam scans over or near the servo and clock pits, and the focus servo is caused to vary over a range to locate the focus position which provides a maximum read signal from the clock pits.
Re-calibration of sophisticated, high density data readout systems is often initiated automatically. For example, the temperature of the optical data drive may be monitored, and re-calibration initiated whenever the system detects a temperature change greater than a predetermined amount. The detected signal quality can be measured, and re-calibration be initiated whenever the signal quality has fallen below a predetermined threshold, or has degraded by an excessive amount.
Some known systems, and proposed systems, include recording of reference marks between the servo field and the data field of a frame, or as part of a preamble in a CCS system. These marks are recorded when data are recorded, and are used during readout to set a threshold value for detection of marks (ones) as opposed to blanks (zeroes) in the following data field. However, these reference marks have not been used for setting or optimizing focus offset values.
These prior art techniques can provide fairly accurate focus optimization if sufficient time and circuit power are available, so that the focus mechanism can be stepped over a portion of its range to each side of the setting at which a four-quadrant detector or the like indicates an optimum signal--generally, the highest peak and steepest skirts when scanning over a recorded mark. However, the clock pits are not closely representative of the data marks whose reading is the goal of the whole process, because the control marks may be smaller than the data marks, may have an effective position within or adjacent the recording layer different from the data marks, and may have a larger clear adjoining space than is true of the data marks.